The allyl ether as a protecting group in carbohydrate chemistry. Part 10. Synthesis of 3-O-(β-D-galactopyranosyl 3-sulphate)-2-O-hexadecanoyl-1-O-hexadecyl-L-glycerol, ‘seminolipid’
3-O-Allyl-L-glycerol was converted, by way of the 1-O-trityl derivative, into 3-O-allyl-2-O-(but-2-enyl)-L-glycerol, which was alkylated with hexadecyl bromide and sodium hydride in NN-dimethylformamide. The product was converted, by the action of potassium t-butoxide in dimethyl sulphoxide, into 1-O-hexadecyl-3-O-(prop-1-enyl)-L-glycerol which was alkylated with ‘crotyl bromide’ and the product was hydrolysed to give 2-O-(but-2-enyl)-1-O-hexadecyl-L-glycerol. This was condensed with acetobromogalactose to give the β-galactoside which was converted into crystalline 2-O-(but-2-enyl)-1-O-hexadecyl-3-O-(3,4-O-isopropylidene-β-D-gaiactopyranosyl)-L-glycerol. This, on benzylation followed by acidic hydrolysis, gave 3-O-(2,6-di-O-benzyl-β-D-galactopyranosyl)-2-O-(but-2-enyl)-1-O-hexadecyl-L-glycerol which was converted, by way of the O-dibutylstannylidene derivative, into 3-O-(3-O-ally) 2,6-di-O-benzyl-β-D-galactopyranosyl)-2-O-(but-2-enyl)-1-O-hexadecyl-L-glycerol. Benzylation of this compound and subsequent treatment with potassium t-butoxide in dimethyl sulphoxide gave 3-O-[2,4,6-tri-O-benzyl-3-O-(prop-1-enyl)-(β-D-galactopyranosyl]-1-O-hexadecyl-L-glycerol which was acylated with hexadecanoyl chloride in pyridine. The prop-1-enyl group was cleaved by the action of mercury(II) chloride to give crystalline 3-O-(2,4,6-tri-O-benzyl-β-D-galactopyranosyl)-2-O-hexadecanoyl-1-O-hexadecyl-L-glycerol. This compound was sulphated with the pyridine–sulphur trioxide complex and the benzyl groups were removed by catalytic hydrogenolysis in glacial acetic acid to give ‘seminolipid’ with properties similar to those reported for the natural material. ‘Desulphato-seminolipid’[3-O-(β-D-galactopyranosyl)-2-O-hexadecanoyl-1-O-hexadecyl-L-glycerol] was also prepared by catalytic hydrogenolysis of 3-O-(2,4,6-tri-O-benzyl-β-D-galactopyranosyl)-2-O-hexadecanoyl-1-O-hexadecyl-L-glycerol.